High-frequency-measuring system



Feb. 2s, 11928. l 1,660,405

H. A. AFFEL HIGH FREQUENCY MEASURING SYSTEM Filed Nav. 2l. 1924 I f2 ,ng/@6MM aml Patented Feb. 28, 1928.

. UNiTEu STATESgin'rizirr lorries.

HERNAN A. AMEL, or nAPLEwoon, NEW JERSEY, AssiGNon To AAmniiraicAvN TELE- PHONE. AND TELEGRAPH COMPANY, A CORPORATION or NEW ironic.

HIGH-FREQUENCY-HEASURNG SYSTEM.

Appiicatio. mea November 2'1, 1924. serial No. 751,407.

This invention relates to high frequency measuring systems, and particularly to an arrangement adapted not only to measure the impedance of a circuit at high frequencies but also to determine the magnitudes of the component elements of such impedance.

In my copending application, Serial No. 751,406, filed November 21, 1924, there.Y 1s

l0 'disclosed a method and means for measuring high frequency impedance, the method consisting in balancing an unknown direct c urrent potential by a similar known potential the ldirect current potential being produce by rectification or other method of conversion of unknown and known currents, respectively, of frequencies at which it is desired to measure. By translating the alternating currents into proportional direct current potentials, and by opposing these potentials in a circuit having a galvanometer, a visual indication of the balance of the high frequency currents is thereby afforded and A it is thus possible to eliminate serious errors which were introduced into such measurements by the testing apparatus heretofore used.

The object of the present invention which embodies .the general principle set forth above is to measure impedance and also 'to determine the resistance and the reactance 1n Fig. 1 operates is as follows:

components thereof.

Other objects of this invention will be apparent from the following description when read in connection'with the attached drawing, of which Figure 1 is aform of embodiment ofthe invention for determining the magnitude of an unknown impedance and Fig. 2 shows an arrangement for separate' determination of the components of an impedance.

In Fig. 1 of the drawing, X represents an impedance whose value 1s unknown, andv which it is desired'to determine.v While any impedance may be Measured by this arrangement, for the urpose of illustration we will assume that t e impedance whose value is to be determined is that of aline circuit. The elements, R, L, C, represent resistance,

inductance and capacity, each element being referably variable and of known value.

iThese elements, taken together, constitute the known impedance. The known and the unknown im edance are connected with the winding 2 o the transformer 3. This windturn,'is connected with the junctioiipoint of ing is divided into two arts, 4 aiid 5 which are preferably equal. T e junction point of the windings 4 and 5 is connectedj with the source 6 of alternating current vwhich, in

the known and the unknown impedances. y The connection with the source 6 of alternating current includes the windin 7 of the transformer 8 which has a secon ary Winding by means of which an alternating volt- 05 t age 1s applied across the impedance 10 of the balanced homodyne detector. This balanced detector is of the type shown inthe patent to Carson, No. 1,449,382, which issued on March 27th, 1923. The secondary 11 of transformer 3 is connected with the input side of the Vdetector comprising the vacuum tubes V1` and V2. The output sides of the tubes are connected in such manner with the differential galvanometer 12 that the homodyne current appliedy across the impedance 10 produces no effect uponthe apparatus connected in the output circuit, assuming that the apparatus' is roperl balanced. To insure against the el ect` ofliigh frequenc current in the output side 'affecting the ga vanometer 12, Acondensers 1 3 and 14 are conlieclet across the plate circuit to shunt any 1g re uenc currents assin throu h the detectorlubesl, p ,g g

The manner in which the system shown The high frequency testing voltage 6 isvv applied as stated between the junction point of the windings 4 and 5 and the junction point 15 between. the known and unknown, impeda-nces'. This high frequency voltage is also applied Athrough the windings of transformer 8 across the impedance 10^con nected with the common filament conductor of the vacuum tubes V1 and V. An unbalance betweenvunknown impedance and theV known impedance consisting of the elements R, L and C, will cause a resultant How o f current through the windings 4 and 100 5, which resultant will cause a otential to be induced in the winding A11 o transformer 3 which will be. a plied' to the grids of the vacuum tubes 1 and V2 simultaneously with 'the application thereto ofthe Asaine 105 high frequency potential from the same source 6. These frequencies will beat together, and the current resulting therefrom will alect the differential gelvnometer 12. Thisl galvanometer, by` its` ability to -indi- 110 cate plus or minus deflections, will afford a relative indication of the direction of the unbalance and will aid in balancing. Furthermore, it will be seen that by the series connection of the Source of alternating current with that part of the circuit containing the known and unknown impedances and also with the part of the circuit containing the detector and the indicating instrument, the phase relations are maintained constant between the bridge and the detector and by proper preliminaryv adjustment-s, the detector may be operated` at its maximum sensitivity at all times. If the currents flowing through X and R, L and C are not in phase, the potentials induced by them'` across the grids V1 and V2, respectively, will not be in phase. The impedance 10 also causes a potential acrossthe grids of V1 and V2. This added to the'potentials induced from transformer 11 results in total potentials across V,L and V2 which are not equal. Consequently the differential galvanometer will not register zero until the potentials across the known and unknown impedances are-both equal and in phase. By varying the elements R, L and C in the adjustable branch of the measuring circuit until a condition of balance is indicated by the differential galvanometer 12, the magnitude of the unknown impedance X may be determined.

The arrangementlshown in Fig. 2 is de signed for the separate determination of the resistance and the reactance components of the unknown impedance. Its voperation depends upon the fact that for a homodyne detector two separate conditions may induce zero galvanometer deflection, 1) Awhere the phases between the input and the homodyne source are at 900, and (2), where the hases between input and homodyne supp y are other than 90o and the currenty input is zero. The arrangement shown in Fig. 2 shows a system similar to that shown in Fig. 1 in'which a. differential transformer forms the basis of comparison of the currents. In this arrangement, the variable known reactance L and C is inserted in series with the unknown impedance X in one branch circuit, and the Variable known resistance is connected in the other branch circuit. The winding 2 lof the transformer 3 comprises, as in Fig. 1, two parts, vpreferably equal, designated 4 and 5. The variableknown inductance and capacity andthe unknown impedance are connected with one of the terminals of the winding 4, and the variable known resistance R is connected with one of the terminals of winding 5. The junction of windings 4 and 5 is connected with the source of high frequency alternating eurent 6, which in turn is conynected with'the junction of the branch circuits, one containin the known vinductance and capacity and t e other containing the known resistance. This connection with the high frequency generator 6 also containsra winding of transformer 8 by means 'of which the high frequency homodyne current is connected with the impedance 10 of the balanced detector. The voltage applied across this imp ance will of course be proportional to the current iiowing from the generator 6 through the bridge arms com prising the windings Lland 5 and their as-v sociated elements. The source 6 is directly connected, by means of conductor 16, with the impedance 17 of a second balanceddetector comprising the tubes V and V4. The secondary of transformer 3 consists of two windings or two groups of windings, one

-of which` is connected with the balanced detector containing-tubes V,l and V2 and the other of which is connected with the detector containing tubes V3 andV V4. Each of these detector larrangements is connected as in Fig. 1 with a .differential galvanometer. The galvanometer 12 indicates complete balance of the unknown impedance, and the galvanometer 18 indicates simply reactance balance. y

The manner in which the reactance and resistance components of the unknown impedance are determined separately is asfollows: `The reactance component is first determined by a balance obtained on a galvanometer 18 with the circuit connected to vacuum tubes' V-3 and V-4 and making useico action, provide a. 90@ phase relation, the bal- 'ancincr of the homodyne detector circuit will therefore be accbmplished if the currents of circuits 16 and the effective current of circuit 4-5 are in phase. Since circuit 16 is practically pure resistance, the circuit including windings 4 5 must also be made practically pure resistance. It will be noted y that the circuit containing resistance R and winding 5 is a pure resistance circuit and plays no part in determinino' this condition. Infact, it will be desirable 1n the operation of the bridge to disconnect circuitR or arrange that the resistance is set at a very high value. The remaining operation, that of in'- suring the balance of the differential galvaf nometer 18, is then brought about by manip-7' ulating variable condenser C or Variable inductance L until the reactance ofthe unknown impedance is counterbalan'ced; in`

other words, establishing a condition -of resonance in the circuit, 1n which Case the circuit will, for the testingv frequency,lact like a pure resistance. The necessa condition of balance will then be estab ished and the known reactance, i. e., the value o C or L, will be the totall reactance of the unknown impedance X. The reading will, however, be reversed in sign. c

Maintaining now this condition of resonance throu h the circuit, the` resistance zoy component o the unknown impedance is determined by the manipulation of the vari able resistance R and obtaining a balance in the galvanometer circuit 12. The series connection of the homodyne supply 6 will prevent the attainment of a balance under any condition other than the alternative set forth heretofore, namely where the phases between input and homodyne supply are not and the input current is zero. The effective input current as provided by winding 11- is of course dependent now on the establishment of zero flux through the transformer winding which is in turn dependent' on the balancing of the currents in windings 4 and 5 of the primary circuit. T his will take place when the resistance R 1s equal to theresistance of the unknown impedance and, as .previously noted, will be determlned by the zero indication of galvanometer l2.

It will accordingly be seen that by means i of the networks'shown on these figured that it is possible to measure quickly the high frequency impedance of any circuit or apclosed as embodied in particular forms it is to be understood thatit is capable o embodiment in other forms without departing from the spirit and scope of the appended claims. What vis claimed is: l 1.- In an impedance measuring system, the combmationwith a source of high frequency potential of a measuring circuit comprismg two branches, one containing an unmown impedance in series with a known capacitance and inductance, and the other containing a known resistance, and a plurality of detecting circuits individual to and connected with the said branches of the: measuring circuit,I each having a differential galvanometer connected with the output side thereef. e

2. In an impedance measuring system, the

jtwo branches, one containing an unknown impedance in yseries with a known capacltance and inductance, and the other containing a known resistance, a transformer, the primary winding of which is connected with both branches ofthe measuring circuit and with said source of potential, a plurality of balanced homodyne detector circuits individual to and connected with the secondary' windings of the said transformer, each comprising two vacuum tubes and a differential galvanometer connected with the output circuits thereof, one of said detector circuits measuring the reactance component of the unknown impedance and the other detector circuit measuring the total impedance. A

3. In an impedance measuring system the combination with a Wheat/stone bridge network comprising two ratio arms `constituting the primary winding of a transformer,l

and third arm containing -an unknown imance in series. with a reactance, and the l ourth arm containing a variable resistance, of av source of potential connected between the Vmidpoint' of the winding constituting the ratio arms and the junction of the said third and -fourth arms, and an indicating circuit comprising a balanced detector hav- I ing two vacuum tubes symmetrically con-` nected with the secondary winding of the said transformer and with the said' source of potentlal and having a differential indieating device connected with the output of l the said detector.- v

4. In an impedance measuring system the combination with a Wheatstone bridge network having two ratio arms, a third arm containing ah unknown impedance in se-l ries with a variable reactance and a fourth. arm containing a variable resistance, of an indicating circuit coupled with the said network containing a balanced detector andoa differential ind eating device, and a source of high frequency current connected with the sald network and the said balanced detector. y

5. In an impedance measuring system the combination with a transformer havin a primary and a plurality of secondaryy wlndings, of a branch containing an unknown -winding and the junction of the Impedance in series with a variable reactance connected with one terminal of the pril mary winding, a second branch containing a. vvariable resistance connected with the other terminal of the said primary' winding, a source of high frequency potential connected with the midpoint ofthe rima rst and second branches, a plurality of indicating circuits each containing a balanced detector y and adiderential indicating device, each indicating circuit being connected with :isec-` a homodyning current independent of the ondary Winding of the said transformer, current in the said branches. v means to apply to oneV of said indicatin In testimony whereof, I .have signed my 1 circuits a homodyning current proportiona name to this specication this 20th day of ,5 to the current flowing through the said N0\"*ember,1924.

rst and second branches, and. means to ap-y y l ply to the other of said indicating circuits HERMAN A.v 

